Structured packings have long been used as mass transfer contacting elements for such purposes as conducting distillation and absorption processes. A key advantage of structured packings is that they provide a large surface area for mass transfer while at the same time exhibit a low pressure drop over a wide operational range. For instance, such structured packings are advantageously utilized in the cryogenic distillation of air to lower pressure drop within a low pressure column of an air separation unit also having a high pressure column linked to the low pressure column in a heat transfer relationship. The lower pressure drop within the low pressure column with the use of such structured packings, as opposed to trays, decreases the pressure to which the incoming air is required to be compressed and therefore, also the electrical power consumption. Additionally, such structured packing is used in an argon column connected to the low pressure column to separate argon from oxygen with a sufficient number of stages of separation that the argon product contains substantially no oxygen.
Structured packings are formed by a plurality of corrugated sheets arranged in a side-by side relationship so that the corrugations in adjacent sheets cross one another. Where structured packing is used in distillation, the structured packings are arranged in vertical groupings of the packing or layers within a distillation column and are fed from the top by a liquid phase of a mixture to be separated and from the bottom by a vapor phase of the mixture. Due to the crossing of the corrugations, the liquid phase tends to spread out across the packing and descend as a film of the liquid. The vapor flows upwardly through the corrugations and also tends to spread out through the packing and contact the descending liquid. The intimate contact of the vapor and the liquid produces a variation of the liquid and vapor mixtures that tends towards equilibrium. As a result the descending liquid will become evermore concentrated in the less volatile components of the mixture to be separated as it descends through the packing and the vapor will increasing become evermore concentrated in the more volatile components of the mixture as it ascends through the packing. Structured packing is provided in layers in which the sheets in one layer are oriented at right angles to the other layer to further promote liquid and vapor mixing.
In structured packing, as well as any mass transfer contacting element, flooding represents a limitation at which the packing will no longer function. Every structured packing has a hydraulic flooding capacity in which the pressure drop starts to rise rapidly. The reason for this is that as the liquid flow in a downward direction through the packing is impeded as a result of excessive upflowing vapor, the liquid begins to build up at the interface between packing layers resulting in the rapid rise in pressure drop leading to flooding. It is to be noted that such pressure drop and resulting liquid holdup between layers is always greater in the interface between the layers of the packing than in the body of the packing because the channels for the flow formed between the corrugations, as between layers of packing, are not aligned. In any case, at about the same time an operational flooding point also begins to be reached at which the mass transfer efficiency of the packing rapidly decreases or in other words, the HETP (height equivalent to a theoretical plate) rises rapidly. The flooding limit for a particular structured packing, conducting a particular distillation, represents a limitation on the flow rate through the distillation column which is addressed in large part by proper selection of the density of the packing and/or providing a column diameter that will produce a sufficiently low vapor rate that flooding will never occur. However, as the density is decreased, the efficiency of the packing also decreases to in turn increase distillation column height requirements. Column sizes and diameters will have a direct effect on the fabrication costs of the plant. Consequently, it is important that the hydraulic flooding capacity of a particular structured packing be as high as possible.
As mentioned above, in conventional packing, the corrugations cross the packing at a constant acute angle with respect to the edges of the packing sheets. As between packing layers, the angle of the corrugations produce an abrupt change in flow direction of both the vapor and the liquid which acts to limit hydraulic capacity. The prior art has provided structured packing that has been designed with end or edge modifications to the corrugations to reduce the abrupt change in flow direction of the vapor to enhance the hydraulic capacity of such structured packing.
An example of a structured packing having an enhanced hydraulic capacity is disclosed in U.S. Pat. No. 5,632,934. In this patent, the corrugations of the structured packing have a central region in which the corrugations are set at an acute angle to opposite edges of the packing sheets and end regions that are at right angles to the opposite edges. The effect of this is to reduce resistance of the vapor as it flows from an underlying layer of packing and enters the layer of packing located above the underlying layer and to allow the liquid fall to an underlying layer with less resistance that conventional packing. As a result, the hydraulic capacity of such a packing is greater than conventional packing not having such an edge modification.
U.S. Pat. No. 6,206,349 discloses another structured packing that is also designed to have an enhanced hydraulic capacity. In the structured packing disclosed in this patent the corrugations within each packing sheets are provided with end or terminal portions that curve from a central section of corrugations towards the edges of the packing sheet so that the change in vapor flow direction is not abrupt and is more gradual. Also provided in this patent is a grid-like structure that could be placed between conventional packing sheets that do not incorporate the curved edge modification with the intent to decrease the change in vapor flow direction as it enters the overlying layer of structured packing.
As will be discussed, the present invention provides a structured packing which, among other advantages, has hydraulic capacity that is greater than the prior art discussed above and further, operational characteristics that are superior than those of the prior art.